The present invention relates generally to a rotary-wing blade of a rotary-wing aircraft, and more particularly, to a cross-sectional shape of a rotary-wing blade. The cross-sectional shape will hereinafter be referred to as a wing section or airfoil section.
In general, various wing sections have been and are being developed for obtaining desired lift and drag forces of fixed wings and rotary wings. Wing sections can be classified by any of the characteristics thereof. A classification by camber of the wing section is illustrated in FIGS. 9(a) through (e). FIG. 9(a) shows a symmetrical airfoil; FIG. 9(b) a positive camber airfoil; FIG. 9(c) a negative camber airfoil; FIG. 9(d) a leading edge camber (leading-edge droop) airfoil; and FIG. 9(e) a trailing-edge camber (rear-loading) airfoil.
Most of the rotary-wing blades of the rotary-wing aircraft of the prior art have airfoil shapes developed by the National Advisory Committee for Aeronautics (NACA), which is predecessor of the National Aeronautics and Space Administration (NASA). Representative examples of airfoils of the NACA are symmetrical airfoil NACA 0012 and cambered airfoil NACA 23012. However, these airfoils have the following inadequacies. Both the subsonic maximum lift coefficient Cl.sub.max and zero-lift drag-divergence Mach number Mdd, required for improving the high-velocity performance of rotary-wing aircraft, of NACA 0012 are low. The NACA 23012 has a relatively high maximum lift coefficient Cl.sub.max. However, its zero-lift drag-divergence Mach number Mdd is low. Therefore the airfoils cannot be expected to contribute to a significant improvement of performance of rotary-wing aircraft .
In comparison, the airfoils proposed in Japanese Patent Publication No. 61-33760 and Japanese Patent Publication No. 1-56960 (corresponding to U.S. Pat. No. 4,416,434) are directed toward performances that are higher. The airfoils have higher maximum lift coefficients Cl.sub.max and zero-lift, drag-convergence Mach numbers Mdd.
FIG. 10 is a graph indicating comparison of the performances of airfoils. The ordinate of the graph represents maximum lift coefficient Cl.sub.max at Mach 0.4. The abscissa represents zero-lift drag-divergence Mach number Mdd. The graph shows points respectively representing the performances of the above mentioned airfoil NACA 0012, airfoil NACA 23012, the airfoil (designated by reference character T) disclosed in Japanese Patent Publication No. 1-56960, and airfoils (SC 1095-R8 and SC 1095) disclosed in Japanese Patent Publication No. 61-33760.
The graph of FIG. 10 indicates that airfoils T, SC 1095-R8, and SC 1095 have higher maximum lift coefficients Cl.sub.max at Mach 0.4 and higher zero-lift, drag-divergence Mach number Mdd than airfoil NACA 0012. The graph indicates also that airfoils SC 1095-R8 and SC 1095 have higher zero-lift drag-divergence Mach numbers Mdd than NACA 23012.
The cross-sectional airfoil section disclosed in Japanese Patent Publication No. 1-56960 is shown in FIG. 11. As shown, the airfoil section is a positive camber section with an up-curved or reflexed trailing edge. The upwardly curved trailing edge is intended to reduce the nose-down (negative) pitching moment. The negative pitching moment is a cause of vibration unavoidable in a positive-camber airfoil and of an increase of the load on the pitch angle varying mechanism.
The cross-sectional airfoil sections SC 1095-R8 and SC 1095, disclosed in Japanese Patent Publication No. 61-33760 are shown respectively in FIGS. 12(a) and 12(b). In the case of these airfoils, the nose-down (negative) pitching moment is reduced by adding trailing-edge tabs to provide an up-turned trailing edge.
The performances of rotary-wing aircraft have been improving steadily in recent years. In accordance with this trend, various high-performance airfoils have been proposed. For example, Japanese Patent Application Laid-Open Publication No. 56-95799 discloses a series of four airfoils designated as VR-12, VR-13, VR-14, and VR-15. The relationship between the maximum lift coefficient Cl.sub.max at Mach 0.4 and the zero-lift drag-divergence Mach number Mdd of each of these airfoils is indicated also in FIG. 10. As indicated in FIG. 10, the airfoils have high values of both Cl.sub.max and Mdd. From this, it can be said that the airfoils have high performance.
The cross-sectional profiles of the airfoils of the series disclosed in Japanese Patent Application Laid-Open Publication No. 56-95799 are shown respectively in FIGS. 13(a), 13(b), 13(c), and 13(d). Similarly as in the case of the other airfoils described hereinabove, upward curves are added thereto in the vicinity of their trailing edges. Thus, the nose-down (negative) pitching moment due to positive camber is reduced in each case.
In general, the pitching moment of a symmetrical airfoil is essentially zero. Furthermore, a symmetrical airfoil is advantageous with regard to its zero-lift drag-divergence Mach number. However, a symmetrical airfoil has the disadvantage of low maximum lift.
In this connection, most of the positive-camber airfoils of the prior art have added positive camber over substantially the entire region thereof from the leading edge to the trailing edge. This feature is intended to increase their maximum lift coefficient. However, in order to mitigate the excessively great nose-down (negative) pitching moment to which these positive-camber airfoils are subjected, they are provided with up-curved or reflexed curves at their trailing edges.
Increasing the angle of up-turn of the airfoil trailing edge may appear to be a measure for further decreasing the nose-down pitching moment. However, increasing the up-turn angle of the trailing edge results in performance-deteriorating effects such as flow separation at the lower surface of the up-turned part of the blade. For this reason, in the case where the allowable value of the pitching moment is small, a major change in the design of the wing section shape becomes necessary. An example of one item of such change is a reduction in the camber.